The general aim of this research is to study the mechanism controlling cholesterol transport between the blood, liver and bile. Since free cholesterol (FC) excretion in bile is the only route for cholesterol removal from the body, this mechanism may be important in hypercholesterolemic states, atherosclerosis, and cholesterol gallstones. Recent studies is fasting subjects have shown, based on kinetic analysis, that cholesterol in an hepatic subcompartment rapidly exchanges (bidirectional transfer) between plasma HDL and biliary micelles. A portion of this subcompartment includes the microsomal site of bile acid synthesis. The major aim of the proposed project is to elucidate the characteristics of this hepatic cholesterol transport system. The effect of chylomicron cholesterol on the transport system and on the hepatic cholesterol subcompartment will be studied in man. HDL 3H-FC will be administered during an endogenous infusion of cholesterol rich chylomicrons. From the bile and blood data, we will determine the changes induced by chylomicrons on cholesterol kinetics by using multicompartmental analysis. In other experiments, the transport of FC between biliary micelles (artificial and autologous) and the liver will be examined. Labeled bile acid/phospholipid (PC) micelles with or without 3H cholesterol will be infused retrograde into the hepatic duct of pigs. Liver (radioautography), hepatic vein blood and bile samples will be obtained to determine the site and extent of FC transport. New experiments will be carried out to determine if a cytosolic FC binding protein mediates the rapid interchange of FC between bile, blood and liver. An assay will be developed using acceptor and donor membranes to measure the ability of various cytosol proteins using acceptor and donor membranes to measure the ability of various cytosol proteins to facilitate FC transfer. Transfer activity will be measured in animals treated with agents that effect biliary cholesterol secretion such as estrogens and ursodeoxycholic acid. Preliminary studies in man have suggested that a PC transport system may be involved in biliary PC scretion. Autologous lipoproteins labeled with 14C PC (palmitic-linoleic) will be administered, bile and blood collected, and the data subjected to multicompartmental analysis. The interrelations between the PC and FC transport systems will be evaluated.